Modulatable power transmission clutch

ABSTRACT

A modulatable power transmission clutch has a rotatable driving member, a rotatable driven member, and slippingly engagable clutch plates between said members which are movable into engagement by a fluid-applied spring-released movable piston having two separate fluid application areas thereon, with one area being larger than the other. Fluid is supplied directly to the smaller piston area by an operator-controlled proportional solenoid valve to regulate clutch modulation. Fluid flow to the larger piston area is controlled by a spring-biased trigger valve which pops open when fluid pressure to the smaller piston area reaches a predetermined level, to thereby effect maximum clutch engagement. The trigger valve operates a dump valve which opens to relieve fluid pressure on the larger piston area when the trigger valve recloses in response to reduction of fluid pressure from the solenoid valve.

BACKGROUND OF THE INVENTION

1. Field of Use

This invention relates generally to modulatable power transmissionclutches and, in particular, to those wherein a fluid-appliedspring-released piston operates on clutch plates disposed between arotatable driving member and a rotatable driven member to effect clutchmodulation.

2. Description of the Prior Art

U.S. Patent application Ser. No. 643,357, filed Dec. 22, 1975 toSchneider et al for "Swing Control for Crane", now U.S. Pat. No.4,070,926, and assigned to the same assignee as the present applicationdiscloses a power transmission system for swinging the upper section ofa mobile crane in opposite directions. That power transmission systememploys a transmission having a pair of alternately actuatable "wet"type prior art transmission clutches therein. Each transmission clutchincludes a rotatable driving member, a rotatable driven member, andslippingly engagable clutch plates between said members. Each clutch ismodulatable between a minimum engaged position and a fully engagedposition by means of application of pressurized hydraulic fluid from anoperator-controlled proportional solenoid valve to a fluid-appliedspring-released movable piston which is embodied in the clutch and actsupon the clutch plates. Such a clutch is modulatable by applying fluidat various pressures to the piston and is able, for example, with aclutch plate diameter of about 9 inches, to transmit dynamic torques inthe range of 4500 to 8000 ft. lbs. and hold static torques equal to orup to 150% of that value. Such a clutch is designed so as to have enoughfriction material area in its clutch plates so as not to create thermalproblems within the clutch and cause distress or failure.

SUMMARY OF THE PRESENT INVENTION

A modulatable power transmission clutch in accordance with the presentinvention is similar to the above-described prior art clutch in that itcomprises a rotatable driving member, a rotatable driven member,slippingly engagable clutch plates between the members and fluid-appliedspring-released movable piston for moving the plates from disengaged tofully engaged condition. In accordance with the invention, however, thepiston has two separate fluid application areas, with one piston areabeing larger than the other (i.e., about 61 square inches for the largerand about 9 square inches for the smaller). Fluid is supplied directlyto the smaller piston area by an operator-controlled proportionalsolenoid valve to regulate clutch modulation. Fluid flow to the largerpiston area is controlled by a normally spring-biased closed triggervalve which pops open when fluid pressure to the smaller piston areareaches a predetermined level (about 200 psi, for example) to therebyeffect maximum clutch engagement. The trigger valve operates aspring-biased closed dump valve within the clutch in such a manner thatwhen the trigger valve opens, the dump valve is moved closed and whenthe trigger valve closes in response to reduction of fluid pressure fromthe solenoid valve, the dump valve opens to relieve fluid pressure onthe larger piston area.

The proportional solenoid valve can be operated to apply pressurizedfluid in a pressure range, for example, between 0 and about 200 psi tothe smaller piston area to effect a high degree of regulation or controlover modulation of the clutch by the operator. When fluid pressure onthe smaller piston area exceeds about 200 psi, for example, the triggervalve opens and the dump valve closes to enable fluid at a pressure ofabout 200 psi or above, for example, to be applied to the larger pistonarea thereby maintaining full clutch engagement and maximum torquetransmission therethrough. When fluid pressure is again reduced to apressure range below the aforementioned 200 psi, for example, thetrigger valve re-closes and the dump valve opens thereby releasing theclutch from maximum torque transmission condition and permitting theclutch to be modulatable between disengaged and fully engaged conditionby application of fluid pressure to the smaller piston area by means ofthe solenoid valve.

A clutch in accordance with the present invention has all the advantagesof a prior art clutch of the same general size and characteristics but,in addition, enables the operator to directly modulate the output torquewithin a range, for example, of from 0 lbs. ft. up to approximately 800lbs. ft. of torque delivery. Since the clutch has two separate pistonareas, with one of the areas being substantially larger than the other,pressurized fluid can be admitted to the smaller piston area at alltimes and produces normal forces to effect engagement of the clutch inproportion to the pressure of the fluid and the size of the small pistonarea. Such force must, however, be sufficient to overcome thecountervailing force of the release spring which urges the piston awayfrom the clutch plates before the clutch becomes engaged and any torquecan be transmitted therethrough. Increase of forces above thecountervailing spring force effects proportional engagement of theclutch and proportional torque delivery. Thus, the operator is able notonly to modulate torque but to do so at relatively high pressures whileachieving minute changes in torque at these high pressures. The triggervalve, which is normally spring-biased to closed position, is actuatedto open position when fluid pressure applied to the smaller piston areaexceeds a predetermined level to admit oil to the larger piston area.This results in a large increase in the force acting to engage theclutch and thereby provides a large increase in torque transmissioncapacity.

A clutch in accordance with the invention has a large capability forstatic torque transmission while still making possible very sensitivemodulation of the torque from approximately 0 to 20% of clutch capacity.The clutch is also rapidly releasable or disengagable. Other objects andadvantages of the invention will hereinafter appear.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view partly in cross section of a power transmissionincorporating a pair of clutches in accordance with the invention;

FIG. 2 is an enlarged cross-sectional view of one of the clutches shownin FIG. 1;

FIG. 3 is a graph depicting the relationship between oil pressureapplied to operate the clutch shown in FIG. 2 and the torque deliveredby that clutch; and

FIG. 4 is a graph depicting the relationship between the oil pressurerequired to operate the clutch and the output rotation speed of theclutch.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows a power transmission system including an engine 25, atorque converter 26, and a transmission 28 have modulatable right andleft wet-type fluid-applied spring-released transmission clutches 30 and31, respectively, in accordance with the invention. As FIG. 2 shows,each clutch 30 and 31 is modulated from non-engaged to a partially orfully engaged condition by means of pressurized hydraulic fluid which issupplied from a reservoir 73 by a pump 72 to normally closedproportional solenoid valves 33 and 34, respectively. The control valves33 and 34 may take the form of similar valves shown in aforementionedpatent application Ser. No. 643,357, issued Jan. 31, 1978 as U.S. Pat.No. 4,070,926, and are understood to have valve spools axially movablebetween a fully closed and fully open position by means of anelectrically operated proportional solenoids 33A and 34A, respectively,which are energized by an electric current signal from a control unit40, also described in aforementioned application Ser. No. 643,357, nowU.S. Pat. No. 4,070,926, which has an operator's control lever 37associated therewith. When control lever 37 is in neutral (shown in FIG.2), the clutches 30 and 31 are disengaged. When the control lever 37 ismoved from neutral toward the L or R positions, the solenoid valves 33or 34, respectively, and the clutches 30 or 31, respectively, areoperated. The amount of operating fluid metered by a valve 33, 34 tomodulate its associated clutch 30, 31 is proportional to the electriccurrent applied to the proportional solenoid valve and is a function ofthe throw of lever 37. FIG. 2 shows that the control valves 33 and 34control the supply of hydraulic operating fluid to their respectiveclutches from pump 72. The two control valves are identical to eachother in construction and mode of operation. In practice, each controlvalve could take the form of control valve 33 shown in FIGS. 9 and 10 ofU.S. Pat. No. 4,070,926. Control valve 33 comprises a valve body 185having an oil inlet port 188 which is connectable to pump 72, an oiloutlet port 189 which is connectable to the clutch being controlled bythe valve, and an outlet 190 which is connectable to the sump orreservoir 73.

As FIG. 1 shows, transmission 28 comprises a housing H for thetransmission clutches 30 and 31 and into which a transmission inputshaft 1 extends and from which a transmission output shaft 2 extends.Input shaft 1, which is mechanically connected to torque converter 26and driven thereby in the direction of the arrow 7, is supported forrotation on housing H by roller bearing assemblies 3 and 4. Output shaft2, which is rotatable either in the direction of arrow 8 or arrow 9,depending on whether clutch 30 or 31 is being modulated, is supportedfor rotation on housing H by roller bearing assemblies 5 and 6.

As FIG. 1 shows, a gear 11 is spline-connected at 12 to input shaft 1and is in constant mesh with a gear 13 which is mounted for relativerotation on output shaft 2 by a roller bearing assembly 14. Clutch 30has two relatively rotatable sections 30A and 30B and clutch 31 has tworelatively rotatable sections 31A and 31B. Gear 13 is rigidly connectedas by welding at 15 to section 30A of clutch 31. A gear 17 isspline-connected as at 18 to output shaft 2. Gear 17 is connectedthrough an idler gear 20 to a gear 21 which is mounted for relativerotation on input shaft 1 by a roller bearing assembly 22. Gear 21 isrigidly connected as by welding at 23 to section 31A of clutch 31.

Each clutch 30 and 31 is modulatable between fully disengaged, throughpartially engaged to synchronization or lockup, whereafter itautomatically sequences to full torque capability. When control lever 37is in neutral, both clutches are fully disengaged. However, when controllever 37 is moved from neutral to either right or left (R or L), thenclutch 30 or 31, respectively, will be modulated. Referring to FIG. 1,the power flow through transmission 28 is as follows. Assuming thatinput shaft 1 is rotating in the direction of arrow 7 and that neitherclutch 30 nor 31 is actuated, output shaft 2 is stationary. Assumingthat clutch 30 is engaged, power flows from input shaft 1, gear 11, gear13, clutch section 30A, clutch section 30B, and output shaft 2, whichrotates in the direction of arrow 9. Assuming that clutch 31 is engaged,power flows from input shaft 1, clutch section 31B, clutch section 31A,gear 21, gear 20, gear 17, and output shaft 2, which rotates in thedirection of arrow 8.

FIG. 2 is an enlarged cross-sectional view of right transmission clutch31 and shows the manner in which it is associated with gears 11 and 21and input shaft 1. Since clutches 30 and 31 are substantially identicalin construction, except that they are reversely mounted with respect toeach other, only clutch 31 is hereinafter described in detail. Clutch 31comprises a set of drive plates 53 and a set of driven plates 51. Thedriven plates 51 are spline-connected and axially slidable with respectto a hollow cylindrical housing 54 of clutch section 31A which is weldedat 23 to gear 21. Thus, gear 21, housing 54, and the driven plates 51rotate as a unit when clutch 31 is engaged. The drive plates 53 ofclutch 31 are spline-connected and axially slidable with respect to ahub 60 and the hub in turn is spline-connected as at 61 to inputshaft 1. The drive plates 53 are interposed between the driven plates 51and are axially slidable on the hub 60 so that when the plates 51 arebeing driven as a result of frictional engagement with the drive plates53, rotation of shaft 1 and hub 60 effects rotation of housing 54 andgear 21.

As FIG. 2 shows, means in accordance with the invention are provided inclutch 31 to force the drive plates 53 and the driven plates 51 intoface-to-face engagement with a desired degree of force so that power(torque) may be transmitted therebetween proportional to the amount ofpressure applied to force the plates together.

Such means comprise an annular piston 70 for cooperation with the clutchplates 51 and 53 and an annular piston housing 52, both of which aresupportably mounted on and rotatable with shaft 1. Piston housing 52which has a central opening 80 for receiving shaft 1 is secured in afixed axial position on the shaft by entrapment between a shoulder 81 onthe shaft and a snap-type retaining ring 82 which engages an annulargroove 83 on the shaft. Piston 70 is axially slidable on shaft 1 ashereinafter explained, between the fully disengaged position shown inFIG. 2, wherein it is urged by means of a compression-type piston returnspring 84 and other positions leftward of that shown in FIG. 2 whereinit engages the endmost clutch plate 53. Piston 70, which has a centralopening 86 for receiving shaft 1, is provided with an opening 87 forreceiving a dowel pin 88 which is mounted in an opening 89 on pistonhousing 52. The dowel pin 88 maintains piston 70 aligned with pistonhousing 52 as the piston moves axially. The piston return spring 84 hasone end disposed in a recess 90 in the piston 70 and has its other endengaged with a spring retainer 91 which is mounted on clutch hub 60. Theouter face of piston 70 is provided with an annular projection 92 forengagement with outermost clutch plate 53.

Piston 70 and piston housing 52 cooperate with each other to define twoseparate piston areas A and B on piston 70. Thus, piston housing 52 hasa circumferential outer projection 71 and a concentrically arrangedannular inner projection 76 which define a cavity or chamber 98 forreceiving the larger outermost piston area B of piston 70. The innerannular projection 76 is spaced from shaft 1 and cooperates with piston70 to define a smaller chamber 97 for inner piston area A on piston 70.Fluid-tight seals in the form of annular rings 94 and 95 are provided toseal the two separate piston areas and chambers from one another. Forexample, seal 94 is provided between the outer flange 71 of pistonhousing 52 and the outer circumferential edge of piston 70. Seal 95 isprovided in a groove on inner projection 76 of piston housing 52 andengages the piston 70 to form a seal between the two piston areas A andB. In the embodiment shown, smaller piston area A is on the order of 9sq. inches and outer piston area B is on the order of 61 sq. inches. Aseal 95a is provided between shaft 1 and piston 70 to seal chamber 97.

An axial passage 75 is provided in shaft 1 for transmitting hydraulicfluid for operating piston 70 from solenoid valve 34. Passage 75communicates through a radially extending passage 100 in shaft 1 withthe chamber 97 in which smaller inner piston area A is located. Passage75 also communicates through a radially extending passage 102 in shaft 1with a valve assembly 104 hereinafter described in detail which suppliesfluid to the chamber 93 for the larger outer piston area B of piston 70.

The valve assembly 104 is disposed in a cylindrical radially extendingbore 105 in piston housing 52. Valve assembly 104 comprises a hollowcylindrical valve seat member 106 having a central passage 107therethrough. The valve seat member 106 is engagable by a cylindricaltrigger valve member 109 which is slidably mounted in bore 105 abovevalve seat member 106. The valve seat member 106 is secured in positionagainst displacement by means of a dowel pin 110 which extends throughan opening 111 in piston housing 52 and an opening 112 in the valve seatmember 106. Trigger valve member 109 controls opening and closing of afluid inlet to passage 125 between passage 75 and chamber 93 for largerpiston area B. The valve assembly 104 also includes a dump valve member115 which is axially slidable in a bore 116 in a dump valve sleeve 118which is mounted in bore 105. Dump valve 115 is urged downwardly againsttrigger valve 109 by means of a compression spring 119 in bore 105 whichbears against a flange 120 at the upper end of the dump valve sleeve 118and a flange 121 near the lower end of the dump valve 115. The axialbore 116 in dump valve sleeve 118 communicates with the interior ofclutch housing H which is understood to be filled with oil. The bore 116is intersected by an exhaust passage 124 which communicates between thebore and the chamber 93 for the larger piston area B and serves as afluid exhaust passage. When the dump valve 115 is spring-biaseddownwardly as shown in FIG. 2, the exhaust passage 124 is opened andtrigger valve 109 is moved to a position wherein it effects closure ofthe fluid inlet passage 125 to the larger piston area B. When triggervalve 109 is moved upwardly in response to oil pressure in oil inletpassage 75, it moves the dump valve 115 upwardly against the force ofspring 119 and causes the dump valve to close the fluid exhaust passage124. The trigger valve 109 has a built-in hysteresis which makes it anastable valve. This function is accomplished by means of the two areas Dand E at the face of the trigger valve 109. When the valve 109 issealing as shown, the pressure works on area D, but as soon as valve 109opens, the pressure works on areas D and E, creating a larger force todrive the valve full open.

OPERATION

Clutch 31 operates in the following manner and it is to be understoodthat clutch 30 operates in a similar manner when it is actuated.Movement of control lever 37 by the machine operator from the neutralposition shown in FIG. 2 rightward toward position R causes opening ofsolenoid valve 34 in proportion to the amount of lever movement. Fluidis supplied under pressure by pump 72 from reservoir 73 throughproportional valve 34 to passage 75. Pressurized fluid in passage 75 issupplied directly through passage 100 to chamber 97 and acts against thesmaller inner piston area A, thereby causing leftward (with respect toFIG. 2) movement of piston 70 toward and against the clutch plateswhenever the fluid pressure is sufficient to overcome the opposing forceof piston return spring 84. When the opposing force of spring 84 isovercome, piston 70 can be moved against the clutch plates in responseto the manipulation of the operator's control lever 37. Thus, the clutch31 can be modulated. In accordance with the invention, when lever 37operates valve 34 so that fluid pressure in passage 75 exceeds apredetermined value, the force of the fluid pressure acting on triggervalve 109 causes the latter to pop upwardly to open position therebyopening fluid passage 125 and permitting fluid at the predeterminedpressure level to be applied to the larger outer piston area B. Astrigger valve 109 moves upwardly, it also effects upward movement ofdump valve 115 to a position wherein it effects closure of fluid exhaustpassage 124. In order for trigger valve 109 and dump valve 115 to moveupwardly, the predetermined fluid pressure being applied to triggervalve 109 must be sufficient to overcome the biasing action of spring119. Since exhaust passage 124 is closed and fluid inlet passage 125 isopen, full pressure is exerted by piston 70 against the clutch platesand this pressure is the sum of the forces produced by both the smallerand larger piston areas. When control lever 37 is returned towardneutral, and fluid control valve 34 reduces fluid flow pressure inpassage 75, trigger valve 109 is able to close and dump valve 115 isable to open under the force of spring 119. As this occurs, fluid isdumped from the larger outer fluid application area of piston 70 throughthe exhaust passage 124, and clutch operation is again determined byfluid flow to the smaller piston area A.

Clutch 31 includes all of the normal functions of a wet powershifttransmission clutch of comparable size and in accordance with theinvention includes features that give the operator ability to use it inthe same manner as prior art type dry master clutches in vehicles.

Assuming that clutch 31 is an 8-plate 9 inch diameter wet clutch, itnormally functions when placed in a powershift transmission to transmitdynamic torques in the range of 4500 to 8000 ft. lbs. and to hold statictorques when synchronized equal to that value or up to 150% of thatvalue. Another typical function of a clutch of this size is to haveenough friction material area so that synchronizations of devicesnormally made by use of this clutch will not create thermal problemswithin the clutch and cause distress or failure. In these respects,clutch 31 is equal to all other clutches of this size and friction platearea.

Clutch 31 also has the ability, for example, to modulate output torqueunder direct control of the operator's apply pressure in the range of 0lbs. ft. up to approximately 800 lbs. ft. of torque delivery. Clutch 31makes this possible through the use of the two separate internal fluidapplication piston areas A and B. Since the inner piston consists of asmall area of only 9 square inches, oil pressure admitted to this pistonarea can create normal forces to engage the clutch in proportion to theoil pressure and this area of the piston. Since the force of the releasespring 84 totals approximately 300 lbs. disengaging force, the pressurein the small piston area A must equal or exceed 35 psi before anyinitial clutch torque is transmitted. The pressure on small piston areaA can then be escalated from 35 to 200 psi with proportional changes inthe normal force creating various amounts of torque. At 200 psi of fluidpressure, for example, a normal force equal to a dynamic capability ofapproximately 800 lbs. ft. of torque is achieved. Thus with thisconstruction, the operator may not only modulate torque, but can do soat relatively high fluid application pressures where he has definitivecontrol over minute changes in torque, since such torque changes requirelarge changes in fluid pressure to achieve.

During operation in accordance with the previous paragraph, the fluidapplied to the smaller piston area A is also pushing upward on triggervalve 109. Valve 109 is held down (closed) by the dump valve compressionspring 119 reacting against the dump valve sleeve 118. The spring 119 isso designed as to hold the trigger valve closed and not admit oil to theouter piston area until, for example, 200 psi of fluid pressure isexceeded. When the oil pressure exceeds 200 psi, the trigger valve 109comes off of its seat 106 and admits oil through passage 125 against thelarger outer piston area B. Simultaneously, trigger valve 109 snapsupwardly without regulation and moves dump valve 115 upward to close offthe exhaust passage 124 from the outer piston area B. With exhaustpassage 124 closed, the chamber 93 for larger piston area B fills upwith fluid and provides a large increase in normal force acting on theclutch plates and therefore effects a large change in torque capacity.With a fill pressure of approximately 250 psi being applied on the totalclutch, both the inner smaller piston area A and the outer larger pistonarea B have a combined static pressure of 250 psi to create normal forceto apply the clutch. In addition to this, of course, there arecentrifugal head pressures attained in each piston housing chamber 93and 97 equal to the rotational speed of the piston housing 52 which alsotends to create additional normal clutch force.

Clutch 31 has a large capability for static torque transmission and fordynamic capability and still makes available very sensitive modulationof torques from approximately 0 to 20% of clutch capacity. The releasetimes of the clutch are quite fast, since the fluid from outer pistonchamber dumps through the dump valve exhaust passage 124. When oilpressure is reduced dump valve 115 assumes the down position (shown inFIG. 2), and a large flow path is available for exit of the fluid fromthe outer piston area B. The small piston area A is never exhaustedthrough dump valve 124. Thus, lowering of oil pressure to under 35 psiallows the clutch 31 to release by itself since the force of spring 84is in excess of apply pressures under the value of 35 psi, for example.Centrifugal heads generated by oil acting on the smaller piston area Aare negligible at speeds up to 3000 to 4000 RPM.

Referring to the graph in FIG. 3, the torque output of clutch 31 versusoperator controlled oil apply pressure into the clutch is showngraphically. As the graph shows, clutch 31 provides for very high fluidpressure with a limited torque available during modulation. If theclutch 31 were designed to modulate at torques up to 3000 or 4000 ft.lbs., instead of only 800 ft. lbs., as disclosed herein, the horsepowercapability that the operator would control during slipping wouldprobably be considerably beyond the thermal capability of this area ofthe clutch plates of the clutch disclosed herein. Typical applicationsof clutch 31 are in various machines that need gentle torquemanipulation of the driven device. These would include sensitive swingdevices on cranes, inching movements on farm tractors or other vehiclesand transmissions that need direct relationship from the operator handlecontrol to the sensitivity of the torque output at these low levels.Once the predetermined pressure level of 200 psi (800 lb. ft.) capacity,for example, has been exceeded, the trigger valve 109 and dump valve 115force the clutch 31 to a very high capability whereby it can thenprovide torques to synchronize the clutch and prevent high horsepowerusage continually and thus thermal failure.

As the graph in FIG. 4 makes clear, since the trigger valve 109 and dumpvalve 115 are spring loaded, any carrier or housing 54 that rotatesdevelops an outward centrifugal force due to their weight that opposesthe downward force of spring 119. This force tends to lower the crackpressure (200 psi, for example) when the speeds in the carrier increase.While the weights of the valves 109 and 115 can be arbitrarily madeheavier or lighter, those employed in an actual embodiment were designedto allow operation up to 4500 RPM. At this point, clutch 31 wouldcentrifugally open the trigger valve 109 for admittance of oil pressuresall the way down to 0 psi and thus could not be controllable forpurposes as previously described. Typical applications of clutch 31 andits rotative speed are in the area of 500 RPM to 2000 RPM where crackpressures are still in the usable values of 200 psi down to 160 psi. SeeFIG. 4.

Clutch 31 may be modified and applied in ways other than shown in thedrawings. For example, by sizing fluid inlet hole 125 to orifice intothe outer piston area B a given quantity of oil when opened by thetrigger valve 109 when it is moved at 200 psi, the effect would be totime the filling of the chamber 93 for the outer piston B and thusattain the high dynamic capability over a longer period of time givingin effect a modulated increase, for example, from the 800 ft. pounds upto the 4000 to 6000 ft. lb. range. This would be advantageous in certaintransmission applications.

It is also possible, for example, to tie in two clutches 31, if embodiedin one single carrier back-to-back in a dual design, so that thepressure from the larger piston area B on one clutch is transmittedthrough the carrier to the vented area containing the dump valve 115 inthe other clutch. This would provide absolute control over the twoclutches so that, while one clutch had pressure in the outer piston, theother one could not be supplied with any pressure against its largerpiston area B. This is an advantage in that it internally guaranteesagainst the inadvertent application of two clutches simultaneously in atransmission.

We claim:
 1. In a modulatable power transmission clutch:a fluid operatedmovable piston for effecting clutch operation and having separate fluidapplication piston areas thereon, one of said piston areas being adaptedto have fluid flow directed thereto at a variable fluid pressure wherebysaid clutch is modulatable, the other of said piston areas being adaptedto have fluid flow directed thereto to effect maximum and unmodulatableengagement of said clutch; and valve means for controlling fluid flow tosaid other of said piston areas in response to fluid pressure at saidone of said piston areas, said valve means including a normally closedfirst valve which opens fully to permit fluid flow to said other pistonarea when fluid pressure at said one piston area exceeds a predeterminedvalue and re-closes when said pressure falls below said predeterminedvalue, said valve means further including a normally open second valveactuatable to closed position by movement of said first valve to openposition which closes fully to prevent fluid flow from said other pistonarea when said first valve opens and which re-opens fully to permitfluid flow from said other piston area when said first valve re-closesfully, said valve means also including spring means for biasing saidfirst valve to closed position and said second valve to open position,said first valve including a surface area against which fluid pressureacts which increases in size when said first valve opens to therebyincrease the force with which said first valve moves to fully openposition.
 2. A modulatable power transmission clutch comprising:arotatable driving member; a rotatable driven member; slippinglyengagable friction clutch means between said members for establishing adrive connection therebetween; a movable piston for causing variableengagement of said friction clutch means; said piston having twoseparate fluid application areas; first means for applying a fluid atvariable pressure to one of said fluid application areas of said pistonto cause variable engagement of said friction clutch means and enablemodulation of said clutch, and second means including first valve meansresponsive to fluid pressure at said one fluid application areaexceeding a predetermined pressure to open fully for applying fluid tothe other of said fluid application areas of said piston to increase theengagement of said friction clutch means to maximum and unmodulatableengagement, said second means further including second valve means whichopens fully for releasing fluid pressure from said other fluidapplication area when said fluid pressure applied to said one fluidapplication area decreases below said predetermined pressure and saidfirst valve means re-closes fully, said first valve including a surfacearea against which fluid pressure acts which increases in size when saidfirst valve opens to thereby increase the force with which said firstvalve moves to fully open position.
 3. A modulatable power transmissionclutch comprising:a rotatable driving member; a rotatable driven member;slippingly engagable friction clutch means between said members forestablishing a drive connection therebetween; a movable piston forcausing variable engagement of said firction clutch means; said pistonhaving two separate fluid application areas, with one area being smallerthan the other; first means including a passage for applying fluid atvariable pressure to said one fluid application area of said piston tocause variable engagement of said friction clutch means and enablemodulation of said clutch, and second means responsive to fluid pressureat said one fluid application area for applying fluid to said otherfluid application area of said piston to increase the engagement of saidfriction clutch means to maximum and unmodulatable engagement when saidfluid pressure applied to said one fluid application area reaches apredetermined pressure, and for releasing fluid pressure from said otherfluid application area when said fluid pressure applied to said onefluid application area decreases below said predetermined pressure, saidsecond means including a normally closed trigger valve which opens fullyat said predetermined pressure to connect said other piston area to saidpassage and a normally open dump valve which closes fully to preventfluid flow from said other fluid area when said trigger valve opens,said first valve including a surface area against which fluid pressureacts which increases in size when said first valve opens to therebyincrease the force with which said first valve moves to fully openposition.